Method and apparatus for management of fluids in an underwater storage tank

ABSTRACT

Apparatus and method for managing water or other ballast fluid in an underwater storage tank ( 14 ), used for storing fluids such as oil or CO2, where a dis-connectable inflatable bag ( 16 ) is coupled to the underwater storage tank to capture displaced ballast fluid from the underwater storage tank. A supply line ( 36 ) between the bag and tank provides a closed loop system. Embodiments are shown in which the tank is initially filled with fresh water and multiple dis-connectable inflatable bags are used.

The present invention relates to underwater storage tanks and inparticular, though not exclusively, to means of managing fluids in arigid underwater storage tank, principally managing water in a tankwhich stores oil and discharges oil.

Storage tanks have been used in the offshore oil industry for manydecades to either store the oil itself or aid in the separation of theoil from other compounds found in reservoir fluids such as water andsand. Most of these storage tanks are either onshore or in the hull offloating facilities and hence the density of the oil being as low as 80%of the seawater density does not have any significant consequences.

If the tank is placed under the sea, the buoyancy effect of this oil canbe significant, plus the gap or void space in the tank above the oil isimpractical to be a large volume gas, even under pressure, due to thevery large buoyancy resulting. To replace the void space it is generallyaccepted that water may be used in lieu of gas. Thus we have a situationof water with oil on top and often a very small gas cap associated withany gas which is coming out of solution.

The other practical and environmental aspect is that the water in thetank must be displaced as oil is loaded. The principal options arere-injection into the wells or discharge to sea.

Discharge to sea requires a degree of treatment and filtration to ensurethe quality of the water released to the environment is at safe levels.Re-injection can be performed; however, there is usually formation waterin the oil which will drop out in the tank and mix with the water in thetank. If this existing water is seawater then there is a risk ofprecipitation of scale from the reaction between seawater and formationwater. Chemical inhibition of scale can be performed but the volumesinvolved are large in this case.

Additionally as the oil is offloaded, which can demand high offloadingrates of 30,000-50,000 m³ per day, the water must be replaced in thetank. This places high demands on the seawater filtration and treatmentsystems to accommodate the throughput required.

Further the water that is in the tank may be heated and thus thedischarge to sea will result in the loss of some of the thermal energy,even with heat exchangers being used. Conversely the incoming seawaterto replace the oil being exported may cool down the tank contents whichmake the oil more viscous plus require heating to the targettemperature. The energy required due to the very high specific heatcapacity of water will be significant.

There are principally three methods currently used for storing oil orother fluids subsea:

-   -   A submerged heavy gravity structure tank with sufficient weight        to counteract the buoyancy from a partially or fully empty tank        containing air;    -   A submerged tank, in which the water is exchanged with local        seawater; and    -   A submerged tank with an internal bag to store the oil.

The methodology of using a tank with sufficient weight to counteract thebuoyancy forces of an empty tank, leads to a very large, heavy andexpensive structure. In addition to float out such a structure, which isdesigned to rest on the seabed when empty, necessitates the addition ofballast chambers which further increase the size and cost. Concrete isoften used for part of the majority of such structures with theattendant inefficiency of concrete having a lower density when comparedto steel plus the environmental impact of its use.

The methodology of using a seawater exchange tank to store the oilplaces demands on the filtration system for the seawater entering andleaving the tank plus requires the use of chemical inhibition to controlscale formation as a result of mixing seawater with formation water.

In addition the methodology of using a seawater exchange tank to keepCO2 in liquid form in deep water (i.e. greater than 500 m depth) placesdemands on the management of the seawater, which will become acidic bydissolution of CO2 into the seawater in contact with the liquid CO2.

A more recent methodology is to use an inflatable bag to store the oil,as described in WO2015/110413 and WO2004/037681. This provides seabedlocated storage for crude oil or other fluid, distinguished in that itis comprising a storage section in the form of an oil and waterproofcloth formed as a flexible balloon that can be filled with, store andemptied for a storage fluid, a structure section formed as an externalcasing over the storage section, which structure section is closed inthe upper part such that an upward close volume of size at leastcorresponding to the volume of the storage section is formed, but withopenings to the surroundings in the lower part, an anchoring sectionformed as a substructure between the structure section and the seabed,with means for anchoring to or stable placement on the seabed, and atransfer section comprising pipes and valves for loading and unloadingof a storage fluid, arranged in substance exterior to the upper part ofthe storage. Such an arrangement has the following disadvantages.

-   -   Inspection of the external surface of the bag for damage may be        difficult and inspection of the internals almost impossible.    -   The bag will require a significant elasticity to accommodate the        surface area change when loading.    -   Any penetrating damage to the bag which results in the release        of oil, will require a cap to capture the oil released, together        with a monitoring system to identify such a release and shutdown        the processing.    -   The inside system of such a cap will require close fabrication        scrutiny to remove any weld scars or edges which could damage        the bag.    -   Recover of the bag if damaged, or for inspection, needs to have        a system to ensure the bag can be recovered without damage or        oil release, especially if there is a large quantity of solids        or highly viscous oils in the bottom, which may hinder or        prevent bag removal.    -   Onshore procedures and methods will require to be developed to        safely dispose of the bag contents and clean the insides,        minimising the hazards to people and the environment.    -   Entry to the bag by personnel is unlikely to be allowed on        safety grounds, hence access to inspect the bag without        destruction of the bag will be difficult.    -   Damage, loss of flexibility or stress concentrations may be        caused to the bag by deposition of solids or thicker compounds        attaching to the internals.

Given that oil from the process train(s) usually contains gas, solids,water and a range of different compounds of different pH and viscositythe integrity demands on an elastic bag to store the oil can be onerousand qualification testing processes for the material will have to beextensive to ensure chemical compatibility.

A related invention to WO2015/110413 A1 is EP 3444427 A1, whereby afirst elastic bag is used to store oil with connectivity to a secondsmaller elastic bag which is used to manage pressure variations in thefirst bag via fluid communication. However, the second bag does not havesufficient volume to contain all of the displaced water from the firstbag and the first bag is subject to the considerations as statedearlier. The invention does not state that replacement of either bag isrequired or possible.

A further invention U.S. Pat. No. 4,662,386 describes a toroidal orcylindrical tank to which is attached a bladder arrangement. Thisbladder accepts the displaced water in a similar manner; however, thebladder is a permanent fixture, which places demands on the integrity ofthe material used for chemical and service compatibility and does nothave removability at the core as a key feature of the invention. Thisplaces onerous demands on the bladder material, given the varyingformation water and oil compositions in different reservoirs.

The profile of the bladder is such that any gas particles coming out ofsolution will gather that the top of the bladder when it is more than50% full (i.e. the upper surface of the bladder is convex) and migrateto the sides when less than 50% full (bladder upper surface is concave).Therefore, there is no means of escape for gas coming out of solution,which will lead to increasing buoyancy and stresses on the bladdermembrane. Further, oil particles which are carried in with the waterwill also migrate to the top of the bladder with no means of exit ormeans of testing the fluid/gas mixture composition for quantity.

The gas dissolution and oil particles will also interfere and reducewith the static external pressure on the bladder which would normallyforce the lighter products in the storage tanks to elevations above sealevel during offloading operations.

It is therefore an object of the present invention to provide apparatusfor managing fluids in an underwater storage tank which obviates ormitigates at least some of the disadvantages of the prior art.

It is a further object of at least one embodiment of the presentinvention to provide apparatus for managing the water in an underwateroil storage tank which obviates or mitigates at least some of thedisadvantages of the prior art.

It is an object of the present invention to provide a method formanaging fluids in an underwater storage tank which obviates ormitigates at least some of the disadvantages of the prior art.

It is a further object of at least one embodiment the present inventionto provide a method for managing the water in an underwater oil storagetank which obviates or mitigates at least some of the disadvantages ofthe prior art.

It is a further object of at least one embodiment of the presentinvention to provide a method for managing the water or other displacedballasting fluid in an underwater liquefied CO2 storage tank whichobviates or mitigates at least some of the disadvantages of the priorart.

According to a first aspect of the present invention there is providedapparatus for managing a first fluid in an underwater storage tank forstoring a second fluid, comprising a dis-connectable inflatable bagcoupled to the underwater storage tank to capture the first fluiddisplaced from the underwater storage tank.

The first fluid may be a displaced ballasting fluid. More preferably thefirst fluid is a liquid. More preferably the second fluid is a liquid.Preferably, the first fluid is water. The second fluid may be liquifiedCO2. In a preferred embodiment the second fluid is oil. There may be aplurality of dis-connectable inflatable bags.

In this way, the dis-connectable bags are used to contain the displacedformation/seawater instead of the oil, the contents of which will have asignificantly lower range of trace compounds within. The risk of anoil-leak together with the consequences is greatly reduced, plus a majoradvantage is that the dis-connectable inflatable bags can bedisconnected and towed to shore for inspection and testing. Such regularinspection will give the manufacturers higher confidence in theintegrity and longevity of the dis-connectable inflatable bags plusallow development of a discard criteria.

Further the regular inspection of the replaced dis-connectableinflatable bags allows planning for the final decommissioning minimisinghazards to humans and the environment, which otherwise would beuncertain from a permanently connected bag.

Preferably, the dis-connectable inflatable bag has a first port for theentry and exit of water. In this way, a closed loop system with theunderwater oil storage tank can be made so that water in the underwateroil storage tank is not mixed with seawater outside of the underwateroil storage tank.

Preferably also, the dis-connectable inflatable bag has a second port,the second port being closed when the dis-connectable inflatable bag isattached to the underwater oil storage tank. In this way, the secondport is used to remove gas which has come out of solution or lighterthan water fluids, which would otherwise increase the buoyancy of thegas and on pass said compounds for further treatment. The port alsoallows flushing of the dis-connectable inflatable bag contents in-situin preparation for disconnection and onshore for maintenance and/ordisposal.

Preferably the dis-connectable inflatable bags are arranged such thatthere are top and bottom entrance ports, to allow removal of heavierthan water compounds through one port and lighter than water compoundsfrom the other.

Preferably the dis-connectable inflatable bags are aligned so thatcomplete draining of lighter than water and heavier than water compoundscan occur under the action of gravity through the ports.

In particular, a top port allows lighter than water fluids and gases tonaturally rise by the action of buoyancy out of the dis-connectableinflatable bag for passing back into the process stream.

This port may, subject to the arrangement of the pipework, be open inservice to allow any compounds to automatically rise to a holding tank,which can be periodically emptied into the process stream. The lowerport is usually open to allow water to enter and exit thedis-connectable inflatable bag, but this can be used to reverse flushthe dis-connectable inflatable bag contents to remove any solids. Asolids trap may be incorporated within the pipework prior to entry tothe dis-connectable inflatable bag to capture solids, improving theefficiency of solids management.

The plurality of ports provide a passageway between an inside and anoutside of each dis-connectable inflatable bag. The provision of morethan one port on the dis-connectable inflatable bag, gives theopportunity to flush the dis-connectable inflatable bags, removing anysolids, gas, or fluids other than seawater/formation water.

Preferably, the dis-connectable inflatable bag is arranged adjacent theunderwater storage tank. In this way, the dis-connectable inflatable bagallows a large amount of enthalpy in the system by allowing warmerdisplaced water to re-fill the tank, rather than colder seawater.

Preferably, there are a plurality of dis-connectable inflatable bagsarranged adjacent the underwater storage tank. In this way, selecteddis-connectable inflatable bags can be disconnected and towed into asafe location for inspection and testing. Additionally multipledis-connectable inflatable bags give redundancy in the event of ablockage or leakage.

Preferably, the apparatus further comprises a supply line between a portand a water exit port at the bottom of the underwater storage tank.Preferably the supply line includes one or more valves. In this way,unplanned back flow can be prevented and piping branches can beisolated.

The supply line may include a branch line to divert water to productionfacilities associated with the underwater oil storage tank.

In this way, water and/or produced water from the underwater oil storagetank can be brought to the production facility for cleaning anddischarge or injection.

The supply line may include an intermediate tank, the intermediate tankbeing used to collect and separate oil or sludge which may have enteredthe supply line as the water from the underwater oil storage tank istransferred to the dis-connectable inflatable bag. Preferably, theintermediate tank has at least one take-off port to direct the separatedoil/sludge back to the underwater oil storage tank. More preferablythere are two take-off ports, arranged at a top and a bottom of theintermediate tank, with the supply line arranged to draw from a pointbetween the top and the bottom of the intermediate tank.

According to a second aspect of the present invention there is provideda method for managing a first fluid in an underwater storage tank, usedfor storing a second fluid, comprising capturing the first fluiddisplaced from the underwater storage tank in a dis-connectableinflatable bag.

The first fluid may be a displaced ballasting fluid. More preferably thefirst fluid is a liquid. More preferably the second fluid is a liquid.Preferably, the first fluid is water. The second fluid may be liquifiedCO2. In a preferred embodiment the second fluid is oil. In this way theunderwater storage tank is an underwater oil storage tank. There may bea plurality of dis-connectable inflatable bags.

In this way, the mix of seawater and formation water is not dischargedto sea as the oil is loaded into the underwater oil storage tank.

Preferably the method includes the steps of:

-   -   (a) connecting a supply line between a single port of the        dis-connectable inflatable bag and a water exit port at the        bottom of the underwater oil storage tank;    -   (b) with the dis-connectable inflatable bag deflated, initially        filling the underwater oil storage tank with water;    -   (c) loading oil into the underwater oil storage tank to displace        the water into the supply line;    -   (d) passing the displaced water into the dis-connectable        inflatable bag and storing the displaced water in the bag;    -   (e) discharging oil from the underwater oil storage tank for        export while allowing the displaced water to return to the        underwater oil storage tank from the dis-connectable inflatable        bag.

In this way, the dis-connectable inflatable bag is used to hold thedisplaced water which reduces the risks inherent if the dis-connectableinflatable bag held oil.

Preferably, at step (a) a plurality of dis-connectable inflatable bagsare attached to a branched connection on the supply line, with eachbranch containing a valve and the method includes the step of isolatingone or more dis-connectable inflatable bags. In this way, additionalbags are available for redundancy.

Preferably, the method includes the step of removing an isolateddis-connectable inflatable bag from the supply line. In this way, theisolated bag can be inspected or replaced.

Preferably, at step (d) the underwater oil storage tank is initiallyfilled with fresh water. In this way, when the fresh water mixes withformation water, introduced via the oil, there is a lower risk ofscaling than would occur if seawater was used to displace the oil.Additionally, as the oil is offloaded in step (e), complex filteringsystems are not required for incoming seawater.

Preferably, at step (c) the oil is loaded into the underwater oilstorage tank from a production facility.

Preferably, the method includes the step of returning gas in theunderwater oil storage tank to the production facility. In this way, theunderwater storage tank is kept full of only oil and water.

The method may include the step of returning displaced water from theunderwater oil storage tank via the supply line to a productionfacility. In this way, the displaced water which now has added formationwater can be cleaned, discharged or used for injection.

The method may include the step of pressure monitoring. This can beperformed at the dis-connectable inflatable bag(s), the tank or at anypoint in the pipelines. In this way, the inflation level in the bags andthe tank can be determined.

The method may include passing the displaced water from the underwateroil storage tank through a separator on the supply line. In this way,any oil or solids/sludge which may have passed into the supply line isprevented from entering the dis-connectable inflatable bag. In this way,the risk of contamination of the dis-connectable inflatable bags isminimised.

This method may also include, at step (e), providing continuous exportof oil while separating the oil and water in the underwater oil storagetank. In this way, the tank is used as a long-residence time separatorto improve the quality of the oil but also allowing continuous export.The dis-connectable inflatable bags may be used according to the samemethodology to capture the produced water, but also provide a volumetricbuffer and against process upsets allowing greater management of theprocess system. The storage requirements of the bags may then besignificantly reduced.

The method may further include the step of discharging water from theunderwater oil storage tank through an oil export line. In this way, theoil export line can be flushed.

The management of pressurised and liquefied CO2 in a deep water storagetank, for carbon sequestration, faces similar challenges in thatexchange of the seawater with the natural environmental is undesirablydue to the resulting acidity from carbon dioxide entering the water insolution and reducing the ph, making it acidic. By provision of the sameclosed loop system that ability to manage the water is provided by usingthe dis-connectable inflatable bags to contain the displaced water. Inaddition the provision of piping to the facility from the bags and thesupply line, allows circulation of the water to topsides, where the CO2has an opportunity to come out of solution due to the near ambientpressure and be recovered for re-injection. Additionally the water canbe filtered and treated before being returned to the tank in the samemanner as formation water. The ability to disconnect the bags is vitalin order that the effect of the acidification of the ballast fluid canbe assessed on the bag material in the same manner as oil or formationwater does.

Accordingly, the drawings and description are to be regarded asillustrative in nature and not as restrictive. Furthermore, theterminology and phraseology used herein is solely used for descriptivepurposes and should not be construed as limiting in scope languages suchas including, comprising, having, containing or involving and variationsthereof is intended to be broad and encompass the subject matter listedthereafter, equivalents and additional subject matter not recited and isnot intended to exclude other additives, components, integers or steps.Likewise, the term comprising, is considered synonymous with the termsincluding or containing for applicable legal purposes. Any discussion ofdocuments, acts, materials, devices, articles and the like is includedin the specification solely for the purpose of providing a context forthe present invention. It is not suggested or represented that any orall of these matters form part of the prior art based on a commongeneral knowledge in the field relevant to the present invention. Allnumerical values in the disclosure are understood as being modified by“about”. All singular forms of elements or any other componentsdescribed herein are understood to include plural forms thereof and viceversa.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying Figures, of which:

FIG. 1 is a schematic illustration of apparatus for managing the waterin an underwater oil storage tank according to an embodiment of thepresent invention;

FIG. 2 is the apparatus of FIG. 1 shown with the underwater oil storagetank full of oil ready for export and the dis-connectable inflatablebags filled with the displaced water;

FIG. 3 is a schematic illustration of apparatus for managing the waterin an underwater oil storage tank according to another embodiment of thepresent invention;

FIG. 4 is a schematic illustration of apparatus for managing the waterin an underwater oil storage tank according to another embodiment of thepresent invention;

FIG. 5 is a schematic illustration of apparatus for managing the waterin an underwater oil storage tank according to another embodiment of thepresent invention and

FIG. 6 is a schematic illustration of apparatus for managing the wateror other ballasting fluid in an underwater liquefied CO2 storage tankaccording to another embodiment of the present invention.

Reference is initially made to FIG. 1 of the drawings which illustratesan apparatus, generally indicated by reference numeral 10, for managingthe water 12 in an underwater oil storage tank 14 according to anembodiment of the present invention. An dis-connectable inflatable bag16 is coupled to the tank 14 to capture water displaced from the tank14.

The underwater oil storage tank 14 is a standard rigid constructiontypically located on the seabed below or in proximity to a productionfacility 18. Tank 14 is used to store oil 20 in the produced fluids 24from the facility 18 which is transported to the tank 14 via piping 22.Within the tank 14, due to density differences, gas 26 will rise and isfed back via a line 28 to the facility 18. Produced water 30, sometimestermed formation water, will partially mix with the water in the tankand together collect in the lower portion 32 of the tank 14. An exportline 34 is also provided from the tank 14 to remove the stabilized crudeoil 20 when the tank 14 is full.

Initially the tank 14 is filled with water 12. In the prior art thiswould have been seawater and a complex arrangement of filtering would berequired to treat the seawater in an attempt to mitigate the scalingthat occurs when seawater comes into contact with produced water 30. Inthe present invention, water 12 is preferentially fresh water. A supplyline 36 is arranged between a port 38 at the bottom 32 of the tank 14and a first port 40 into and out of an dis-connectable inflatable bag16. The bag 16 is of a flexible material to create a barrier betweenwater 12 and the surrounding seawater. The bag 16 can be located withinthe vicinity of the tank 14 either located on top, on the side,underneath or on the local seabed. Optionally the bag 16 can be locatedwithin a structure which provides a degree of protection against droppedobjects and/or stabilisation against the effect of wave and current.

The bag 16, supply line 36 and tank 14 therefore provide a closed loopsystem. In this way the water 12 is contained within the system.

Further piping 42 can be provided for a water top-up line from theproduction facility 18 to the tank 14. If fresh water is not availableto top-up the tank 14, then filtered seawater may be used.

Branch piping 44 can also be provided from the supply line 36 to theproduction facility 18 for water 12 and/or produced water 30 forcleaning, discharge, reinjection or storage in the hull as is known inthe art.

By this means the piping 42 and 44 can control the water volume withinthe closed loop, independently of the production facilities.

Valves 46 are arranged on the pipelines (not all shown) to control flowthrough and/or prevent unwanted back flow in the apparatus 10. Pressuresensors 48 may also be located through the apparatus 10 for monitoringpurposes.

Dis-connectable inflatable bag 16 also has a second port 41. Port 41allows the bag 16 to be flushed through via use of first port 40. Thiscan be done to clean out the bag during maintenance or assist inemptying the bag for disposal. These actions can be done offshorein-situ or onshore.

The dis-connectable inflatable bag 16 has connectors 43 on each port,which allow the bag to be removed and re-connected, once the isolationvalves 46 have both been closed.

The bag 16 is shown vertically in order that solids generally fall tothe bottom of the bag 16 and lighter than water liquids and gases riseto the top port. By this arrangement undesirable liquids, solids orgases can be removed from the bag 16 in service by flushing through thesecond port 41. In particular gases and lighter than water liquids canbe flushed back to the facility 18 via piping 45 where they can re-enterthe process stream.

Alternatively the flushing process can be used to displace solids fromthe tank 14 into the bag 16 for recovery and disposal onshore.

Also shown are the connection/disconnect arrangements 43 which allow thebag to be flushed, isolated and then removed for onshore inspection,replacement or prior to decommissioning of the tank 14.

FIG. 1 shows the apparatus 10 in an initial configuration where thewater 12 which is displaced from the tank 14, can be stored indis-connectable inflatable bag 16. Thus a mix of seawater and formationwater is therefore not discharged to sea as the oil is loaded into themain tank 14. If there is a significant quantity of formation water 30,some of the combined water can be passed back to the main facilities foreither re-injection or cleaning to discharge. Generally, the quantitiesinvolved will be the same as the produced water 30 dropout rate.

In use, produced fluids 24 are sent to the tank 14 via piping 22. Onentering the tank 14, the gas 26 will rise and can be piped 28 back tothe facility 18. The oil 20 will sit on top of the water 12, thoughthere may be an emulsion line created therebetween, with any producedwater 30 mixing with the water 30 in the bottom 42 of the tank 14. Asthe volume of oil 20 in the tank 14 increases, the water 12 is displacedfrom the tank 14 through the supply line 36 and into the bag 16. The bag16 will inflate under the introduction of the water 12 with its volumeequaling the volume of displaced water, minus any water which is sent tothe production facility 18, via the branch line 44. In this way, onlywater 12 is contained in the bag 16 and it provides a variable storagevolume in response to the amount of displaced water. Connectors 43 allowthe bags 16 to be dis-connected and replaced with another bag at anytime, by closure of the isolation valves 43.

As the tank 14 fills, the pressure in the tank 14 will reduce due to thedensity of the oil 20 being less than water 12; however, hydrostaticpressure acts on the bag 16 keeping the pressure in the top of the tank14 slightly above external pressure, due to the head of oil 20 inside,which increases as more oil 20 is loaded. The production facility 18will therefore have to pass processed oil, in the form of producedfluids 24, into the tank 14 at or above the seabed hydrostatic pressure.Pressure monitoring 48 can be provided at various points in theapparatus 10, including the storage bag 16.

Valves 46 are provided as require to prevent unplanned backflow andisolate piping branches as required. For example, where part of thewater 12 being displaced from the tank 14 is being filtered off to theproduction facilities 18 to provide re-injection water.

The bag storage 16 thus provides a flexible buffer storage for the waterto allow management of the treatment and rate of injection into wellsand automatically by their presence provides a buffer against pressuresurges or rapid changes due to process interruptions and valveopening/closures.

If sea water is used in the bag 16, as the formation water 30 mixes moreit dilutes the sea water reducing and potentially eliminating scaleinhibitor requirements.

Reference is now made to FIG. 2 of the drawings which shows theapparatus of FIG. 1 with the tank 14 now full of stabilised crude oil 20and the bag 16 containing a majority of the water 12.

In use, once the tank 14 is full, the oil 20 will be discharged to anexport tanker or other export means via export line 34. As the oil 20 isevacuated from the tank 14 by the oil export pumps 58, the water 12 andformation water 30 stored in the bag 16 is passed back through thesupply line 36 into the main tank 14 assisted by the externalhydrostatic head of pressure acting on the bag 16 and the lower staticpressure at the top of the oil in the tank.

The bag 16 will deflate as water 12 is displaced back to the tank 14until the water 12 reaches a level in the tank 14 whereby oil exportwill be stopped.

The rate of offloading is therefore limited generally by the export rateof the oil, rather than the filtration of seawater to replace the oil,if the bag were not present. The rate of offloading can also beincreases by a taller tank to maximise the head differential between theexternal water pressure and that at the top of the oil column.

This method also has a large technical, economic and environmentaladvantage in that the thermal energy of the water 12 is not dischargedto sea and dissipated in the ocean, but passed straight back into thetank 14. Also as water 12 is passed back into the tank 14 the viscosityof the oil 20 is less affected, than if it was colder seawater, whichwould lead to the oil 20 closest to the water interface cooling quickestand hence becoming more viscous towards the end of offloading.

This also helps maintain an efficient thermal balance in the tank 14 andminimises the temperatures changes which have some influence of thefatigue life of the tank structure.

If the quantity of stored water exceeds the oil volume exported theevacuation may continue to allow flushing of the oil export pipelineusing the water. Any oily water received on the tanker is normallydirectly to slops tanks.

Reference is now made to FIG. 3 of the drawings which illustrates afurther embodiment of the present invention by the incorporation of aseparating tank 50 in the supply line 36. An oil particulates returnline 52 is provided from the top of the separation tank 50 to the tank14 and a solids/sludge return line 54 is provided from the bottom of theseparation tank 50 to the tank 14.

This additional tank 50 is used if there is a risk that thesolids/sludge level at the bottom 32 of the tank 14 reaches a levelclose to the water exit point, port 38, near the tank bottom 32 duringoffloading the water 12 into the supply line 36. In addition, if the oil20 level gets close to the exit point, port 38 during loading, some oil20 may accidently be drawn down into the supply line 36.

A tall intermediate tank 50 can be specified which will allow separationof the oil 20 and solids, prior to the water 12 the bag 16. Such a tank50 is illustrated in FIG. 3, with take-offs top and bottom, but thewater 12 has to pass around a baffle plate, maximising its transit timethrough the separation tank 50.

This provides an additional level of operational efficiency andflexibility to the apparatus 10 though with correct monitoring the tank50 is not required.

Reference is now made to FIG. 4 of the drawings which shows a furtherembodiment of the present invention in the form of using multipledis-connectable inflatable bags 16 a-c. Though three bags 16 a-c areillustrated, it will be appreciated that any number and size can be usedto meet the requirements of the subsea tank 14 used.

Supply line 36 from the tank 14 now has a manifold 56 for water 12distribution to each of the bags 16 a-c. Bag 16 c is disconnected toillustrate that the bags 16 a-c can be removed for inspection andreplacement if needed.

More than one bag 16 is used to provide redundancy, allow a greater useof smaller, more readily available designs, plus allow for disconnectionand recovery for onshore inspection or replacement.

Reference is now made to FIG. 5 of the drawings which shows a furtherembodiment of the present invention in the form of using a holding tank52 to capture lighter than water gases or liquids. Like parts to thosein the earlier Figures have been used to aid clarity. The valves on thetop port of the bags 50 a and 50 b are open to allow such fluids to riseby virtue of their buoyancy relative to water into the holding tank 52.

Valve 50 c is shown as closed to prevent backflow into the bag 16 cValve 52 on the piping 45 to the production facility is shown closed,but can be opened to clear the contents of the tank 52 into the processstream.

A further embodiment of the present invention could use thedis-connectable inflatable bag in a continuous oil export scenario. Theoil is exported continuously through a pipeline to a suitable exportroute, allowing the level in the tank to be maintained relativelyconstant at a level in the subsea tank which most benefits theseparation performance.

This reduces the volume requirements for the bag, but still uses the bagfor automatic management via hydrostatic pressure of not only the volumebut pressure in the system. This is particularly important duringproduction upsets such as start-up and shutdown.

In addition it allows greater regulation over the export rate of the oiland by adjustment of the residence time improve the quality of theexported oil.

Reference is now made to FIG. 6 of the drawings which shows a furtherembodiment of the present invention in the form of using dis-connectableinflatable bag 16 in a liquefied CO2 underwater storage scenario. Likeparts to those in the earlier Figures have been used to aid clarity. TheCO2 76 is injected and recovered in liquid form into the tank 14 throughpiping 70 and maintained under pressure using the hydrostatic headacting on the dis-connectable inflatable bags. As the CO2 occupied moreof the tank the bag inflate to accommodate the displaced water or otherballast fluid. The water can be treated and tested by cycling throughthe bag to the topsides facility 78 along the piping 45. The CO2 canthen be released due to the significantly lower pressure at the surface78, then re-injected down the piping 44. Valving 74 is provided tocontrol the flow. The CO2 can be recovered from the tank and injectedthrough piping 72.

In this way, an aspect is in circulating the water to allow the CO2 insolution to escape, but hydrostatic head keeps the pressure down below.

The principle advantage of the present invention is that it provides adis-connectable inflatable underwater bag or multiplicity of, which isused to capture ballast fluid displaced from an underwater fluid storagetank.

A further advantage of the present invention is that it provides adis-connectable inflatable underwater bag or multiplicity of, which isused to capture the water displaced from an underwater oil storage tank.

Further advantages of the present invention are realised as:

The bag substantially forms a ‘closed loop system’ whereby the water inthe tank is not mixed with the seawater outside of the tank, other thanfiltered seawater injected in to top-up any shortfall in the wateravailable within the system.

This “closed loop system” can be used as a very large separator in acontinuous export scenario, giving a higher quality crude product,better water management and a volumetric buffer against process upsets.

The offloading of oil can take place using the water in the bag toreplace the oil as it is extracted from the tank, avoiding the need forcomplex filtering systems for incoming seawater, which has cleanenvironmental benefits.

The bag allows retention of a large proportion of the enthalpy of thesystem (principally heat energy) by allowing the warmer displaced waterto re-fill the tank, rather than colder seawater, thus aiding energyefficiency and minimising environmental impact. In addition the reducedtemperature changes has some beneficial influence on the fatigue life ofthe tank structure, plus maintain the oil at a higher temperaturereducing its viscosity.

The bag also captures any formation water which separates from the oilin the storage tank and hence mixes with the water already in the tank.The ‘closed loop system’ allows management of the chemical interactionbetween the seawater in the tank and the formation water. In additionmonitoring, testing and treatment of the large volume of water can beperformed as part of the operations to re-inject the surplus water intowells.

Additionally, the bag also allows for fresh water to initially be usedin the tank during the installation phase which will mix with theformation water with significantly less risk of scale than seawater.Further, due to the repeated load/unload operations the formation waterwill eventually dilute and replace the seawater, reducing andpotentially eliminating any scale inhibition requirements.

The use of multiple bags for loading of the displaced water allowsselected bags to be disconnected and towed or lifted onto a vessel to asafe location for inspection and testing, thus allowing a greaterconfidence in the longevity and integrity of the selected bag system.Multiple bags also give redundancy in the event of a blockage or leak.

The foregoing description of the invention has been presented for thepurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Thedescribed embodiments were chosen and described in order to best explainthe principles of the invention and its practical application to therebyenable others skilled in the art to best utilise the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. Therefore, further modifications orimprovements may be incorporated without departing from the scope of theinvention herein intended with the invention being defined within thescope of the claims.

1. An apparatus for managing a first fluid in an underwater storagetank, used for storing a second fluid, comprising a dis-connectableinflatable bag coupled to the underwater storage tank to capturedisplaced first fluid from the underwater storage tank.
 2. The apparatusaccording to claim 1 wherein the dis-connectable inflatable bag has afirst port for the entry and exit of the first fluid, to and from theunderwater storage tank.
 3. The apparatus according to claim 2 whereinthe dis-connectable inflatable bag has a second port, the second portbeing closed when the dis-connectable inflatable bag is attached to theunderwater storage tank.
 4. The apparatus according to claim 2 whereinthe dis-connectable inflatable bag has a second port, the second portbeing open when the dis-connectable inflatable bag is attached to theunderwater storage tank to allow fluids lighter than the first fluid torise to a holding vessel.
 5. The apparatus according to claim 1 whereinthe dis-connectable inflatable bag is arranged adjacent the underwaterstorage tank.
 6. The apparatus according to claim 5 wherein there are aplurality of dis-connectable inflatable bags arranged adjacent theunderwater storage tank.
 7. The apparatus according to claim 2 whereinthe apparatus further comprises a supply line between the first port anda first fluid exit port at the bottom of the underwater storage tank. 8.The apparatus according to claim 7 wherein the supply line includes oneor more valves.
 9. The apparatus according to claim 5 wherein the supplyline includes a branch line to divert the first fluid to productionfacilities associated with the underwater storage tank.
 10. Theapparatus according to claim 7 wherein the supply line includes anintermediate tank, the intermediate tank being used to collect andseparate the second fluid or sludge which may have entered the supplyline as the first fluid from the underwater storage tank is offloaded.11. The apparatus according to claim 10 wherein the intermediate tankhas at least one take-off port to direct the separated secondfluid/sludge back to the underwater storage tank.
 12. The apparatusaccording to claim 11 wherein there are two take-off ports, arranged ata top and a bottom of the intermediate tank, with the supply linearranged to draw from a point between the top and the bottom of theintermediate tank.
 13. The apparatus according to claim 1 wherein thereis a holding tank to capture any fluids lighter than the first fluidwhich have entered the dis-connectable inflatable bags, using thenatural buoyancy of those compounds.
 14. A method for managing a firstfluid in an underwater storage tank, used for storing a second fluid,comprising capturing the first fluid displaced from the underwaterstorage tank in a dis-connectable inflatable bag.
 15. The methodaccording to claim 14 wherein the first fluid is water and the secondfluid is oil to provide a method for managing the water in an underwateroil storage tank, comprising capturing the water displaced from theunderwater oil storage tank in a dis-connectable inflatable bag.
 16. Themethod according to claim 15 including the steps of: (a) connecting asupply line between a single port of the dis-connectable inflatable bagand a water exit port at the bottom of the underwater oil storage tank;(b) with the dis-connectable inflatable bag deflated, initially fillingthe underwater oil storage tank with water; (c) loading oil into theunderwater oil storage tank to displace the water into the supply line;(d) passing the displaced water into the dis-connectable inflatable bagand storing the displaced water in the dis-connectable inflatable bag;and (e) discharging oil from the underwater oil storage tank for exportwhile allowing the displaced water to return to the underwater oilstorage tank from the dis-connectable inflatable bag.
 17. The methodaccording to claim 16 wherein at step (a) a plurality of bags areattached to a branched connection on the supply line, with each branchcontaining a valve and the method includes the step of isolating one ormore dis-connectable inflatable bags.
 18. (canceled)
 19. The methodaccording to claim 16 wherein at step (d) the underwater oil storagetank is initially filled with fresh water.
 20. (canceled)
 21. (canceled)22. (canceled)
 23. (canceled)
 24. (canceled)
 25. (canceled)
 26. Themethod according to claim 16 wherein the method includes the step ofdischarging water from the underwater oil storage tank through an oilexport line.
 27. The method according to claim 14 wherein the firstfluid is water and the second fluid is liquefied CO2 to provide a methodfor managing the water in an underwater CO2 storage tank, comprisingcapturing the water displaced from the underwater CO2 storage tank in adis-connectable inflatable bag.